This invention relates to a burst compatible digital signal receiver circuit used in the field of optical communications, including optical subscriber systems, optical LANs, and optical interconnections.
Two-mode differential transfer impedance amplifiers have been used as the automatic threshold control (ATC) circuit in a conventional burst signal compatible digital signal receiver circuit. The configuration and operation of the two-mode differential transfer impedance amplifier has been disclosed concretely in U.S. Pat. No. 5,025,456.
FIG. 1 is a block diagram of the two-mode differential transfer impedance amplifier. The amplifier comprises a photodiode (light-receiving element) PD, a differential input/output amplifier OP, a peak sensing circuit PDC, and feedback resistors Rf1 and Ff2.
The optical signal Pin projected on the photodiode PD is converted into photoelectric current, which is supplied to the (+) input terminal of the differential input/output amplifier OP. The differential input/output amplifier OP amplifies the level difference between the photoelectric current Iin from the photodiode PD and the threshold value supplied to the (-) input terminal. The inverting output of the amplifier is fed back to the (+) input terminal of the amplifier OP via the feedback resistor Rf1. The noninverting output is supplied to the peak sensing circuit PDC.
The peak sensing circuit PDC senses the peak of the noninverting output of the amplifier OP. The sense output is supplied as the threshold value to the (-) input terminal of the amplifier OP via the feedback resistor Rf2.
Namely, the two-mode differential transfer impedance amplifier outputs such a waveform as always has the output voltage in the middle of the pulse width when there is no signal input, by making the transfer impedance of each of the second and later bits in the burst signal twice as large as that of the first bit in the burst signal.
The aforementioned burst signal compatible digital signal receiver circuit, however, has the following problem: because the operating point of the preamplifier fluctuates as a result of fluctuations in the dark current of the light-receiving element or fluctuations in the output offset voltage of the preamplifier, the duty and reception sensitivity of the identification circuit with a fixed threshold value deteriorate.
There has been another problem: when the marking rate of the input signal has fluctuated, or when the same symbols have continued, the output of the peak sensing circuit PDC fluctuates, resulting in the deterioration of the duty and reception sensitivity.